Bolometer for coaxial transmission lines



1954 W. E. WALLER ET AL ,667,618

BOLOMETER FOR COAXIAL TRANSMISSION LINES Filed Aug. 27, 1951 INVENTOR WILL/AM E. WALLE/P STA/VLEVA. JOHNSON BY @904 $1M ATTORNEY Patented Jan. 26, 1954 BOLOMETER FOR COAXIAL TRANSMISSION LINES William E. Waller, Brooklyn, and Stanley A. Johnson, New York, N. Y., assignors to Polytechnic Research and Development Company Incorporated, Brooklyn, York N. Y., a corporation of New Application August 27, 1951, Serial No. 243,884

6 Claims. 1

This invention relates to bolometers for the measurement of high frequency power in coaxial transmission lines.

An object of the invention is to devise an improved arrangement for the mounting of the bolometer resistance units.

Another object is to devise a bolometer structure which is rugged and is easily assembled.

The bolometer of our invention is of the type in which two bolometer resistors are eifectively riphery by any suitable process such as by apconnected in parallel with respect to the high plying conducting silver paint, either by sprayfrequency currents to be measured and are coning or by brushing. This rim adds strength to nected in series with respect to the direct-curthe disc and prevents splitting of the disc.

rent biasing current. On the front face of disc areformed three Other features of our invention include novel electrodes 6, 6a. and 6b, as shown in Figure 2 and structure providing by-passing and blocking more clearly in Figure 3. These electrodes may condensers, and the provision of protective coveralso be formed by applying conducting paint to ings for the bolometer resistors. the front face of the disc 5 over the stippled The preferred form of our invention is illusareas marked 6, 6a and 6b. It will be noted that trated in the accompanying drawing in which: a narrow gap is provided between the-central Fig. 1 is a longitudinal sectional view of the electrode 6 and each of the outer electrodes 6a assembled bolometer structure shown on a greatand 6b. Suitable temperature-sensitive resisly enlarged scale; tors la and lb are connected across these gaps,

Fig. 2 is a view of Fig. 1 taken along the line as shown in Figures 2 and 3. The size and shape 2-2 but omitting the casing; and of the electrodes, and the dimensions of the Fig. 3 is a view like Fig. 2 with certain parts gaps, may be used as electrical matching paramremoved. eters to match the resistors with the character- Figure 4 shows one example of a bridge ciristic impedance of the connected transmission cuit in which the bolometer may be .used. line.

Referring to the drawing, the bolometer is Protective caps 8 are arranged over resistors formed of a tubular outer casing l which is 7a and 1b and these caps are formed of thin coupled to a coaxial transmission line (not domed pieces of insulating material such as polyshown) by any suitable coupling means. A com styrene. The caps are sealed to the face of ductor 2 is mounted centrally within casing disc 5 by lacquer applied at the rim of the caps. and is connected to the center conductor of the The resistors Ta and 11) may be formed of coaxial line by suitable means. Preferably the Wollaston wire (silver jacketed wire) attached inner diameter of casing I is the same as the across the gaps with conducting paint. Lacquer inner diameter of the tubular conductor of the is then applied to the wire and the adjacent coaxial cable, and conductor 2 has the same outelectrode area, leaving the proper length of wire side diameter as the inner conductor of the 00- exposed in the center. The silver jacket is then axial cable. removed by deplating. The desired resistance The casing I is provided with aradial flange value is obtained by alternately deplatlng and la at its outer end having a flat outer face armeasuring until the desired value is reached. ranged normal to the axis of the casing. A cover The resistor elements 1a and lb may also be plate 3 is secured to the flange la by means of formed of extremely thin metallic films deposited a number of screws 4 passing through the plate on the disc 5 across the electrode gaps, the film and having threaded engagement with the having a thickness less than the depth of penefiange la. The cover plate 3 is provided with a tration of current at the operating freque y cylindrical flange 3a which extends over the and for the particular metal employed in the outer edge of flange la. A cylindrical counterfilm. Such films may be formed according to bore 3b is formed at the center of the inner face the process disclosed in Weber et al. Patent No. of plate 3 and constitutes a waveguide beyond 2,529,436, or they may be deposited by thermal cut-off at the end of conductor 2. evaporation according tothe process disclosed Mounted within the space between the flange u in the application of Weber et a1. Serial- N0.

la and the cover plate 3 is a disc 5' of suitable insulating material, such as mica, for support tral hole 5b for receiving another clamping screw referred to later. with a conducting Preferably disc 5 is provided rim 50 formed around its pe- 699,546, now Patent #2586352;

For measurement purposes, the bolometer is connected in the usual Wheatstone bridge circuit, the two resistors la and lb being connected in parallel with respect to the radio frequency currents to be measured and are connected in series with respect to the direct current for biasin the bridge.

One suitable bridge circuit is shown in Figure 4 where arms A, B and C of the bridge correspond to arms H, I2 and t3" of the bridge in. Hansell Patent 2,481,589, and the indicator D corresponds to 16 in Hansell. ilhe bolemeterrunit shown in Figures 1 to 3 of the application iorms the fourth arm of the bridge} and the two bole-m eter elements 7a and lb areshown connected in series with respect to the direct current source E. These elements, however, are connected: in.- parallel with respect to the source of high frequency energy represented at G and supplied to the elements la and 1b in parallel through the center conductor 2 of the coaxial cable and through the condenser Sec-9b; The: end of. unit lb connected to the plug: 13: is connected to the outerconductor 1 of, the: coaxial cable through the: condenser element Il a-41b;

As; shown in drawing, electrode 6a extends cutwardly and surrounds. one of; the holes 5a so that the outer end portionof. the electrode will be. connected. to the flange. la either directly or through an intervening washer. The central electrode 6' is connected to the end of center conductor 2 by means of a blocking condenser formed of a disc of insulating material 9, such. as a disc of mica of a thickness of. abouto.0005 inch,

and having conductive coatings 9a and 9b applied to opposite. faces thereon. The. disc 9 is provided 7 with. a central. opening, aligned with. the: opening.

5b in the disc. 5 and the blocking. condenser is mounted on; the disc 5 by means of spots of lac-- quer 9c applied around. the outer edge: of the disc 9; The. blocking condenser is clamped against the: end of? conductor 2 by means of a clamping screw H3 passing through discs 5 and 9 and. having threaded engagement with the conductor. The blocking condenser is provided to prevent shunting ofthedirect current fromthe b'olometer' elements by the conductor 2:.

The electrode 6b. is efiectively connected to the flange Ia, for high frequency currents by a bypass condenser formed in the same way as. the blocking condenser and involving' athin disc of" insulating" material. H having conductive coat-' ings He and HI) applied on opposite faces. andbeing, secured to the front face: of disc 5 by means. of drops or spots of lacquer He applied. around the.- edge of disc l I.- It will be noted that coating i-l.-a= has contact with the face of flange in while coating, H?) is in direct contact. with the circular end portion of electrode 6b. It will be under-- stood that the thickness of parts 5 to Hb' has been greatly exaggerated inthe drawing.

In using the bolometer' in a Wheatstone bridge,- direct current for biasing the bridge is supplied to the two resistor elements in series by means of a contact 12 applied on therear face of the disc 5 immediately behind the circular part of electrode 6b and being connected to electrode 611 by means of a pair of. connections IZa passing through holes formed in the disc 5. The bolometer isconnected in. the bridge circuit by means of. a plug 13 having engagement with the contact piece I2.

,The, part of electrode. 6a. which surrounds. the clamping. screw tshould. be of increasedthickness correspondingto th thickness of thegblocking and by-pass condensers, or a conducting washer [4 may be interposed between the electrode Ba and the flange la.

While it is preferred to form discs 5, 9 and H ofv mica, other" insulating; material may be used such as glass. Also, while. it is convenient to form the metallic coatings of the metallic paint applied in the manner indicated, these coatings may be applied in other ways, such as by the 7 process disclosed in the Weber et al., Patent No.

We claim'z.

I bolometer' for coaxial transmission lines comprisin a tubular casing having a radial flange at one end and being adapted for connection to a coaxial line at the other end, a conductor mounted at the center of said casing, a cover plate mounted in spaced relation with said flange, a'disc of insulated material interposed between said cover plate and said flange, a central electrode and two outer electrodes formed on'one' face of said disc andfproviding a narrow gap between each outer electrode and the Gentral electrode, a temperature-sensitive resistor connected across eachgap, a blocking condenser connecting said central electrode with the end of said conductor, means providing a conductive connection between one outer electrode and said flange, means for connecting the other outer electrode to an external source of current, and a by-pass condenser connecting said other elec trode to said flange.

2. A bolometer for coaxial transmission lines comprising, a tubular casing having a radial flange at one end and being adapted for connection to. a coaxial lineat. the other end, a conductor mounted at the center of said casing, a disc of insulated material secured to the outer faceof. said flange, a central electrode and two outer electrodes formed on the inner face of said disc and providing a narrow gap between each outer electrode and the central electrode, a temperature-sensitive resistor connected across each gap,- means connecting said central electrode with the end of said conductor through a capacitive react arice of low value at. the frequency of operation,

means providing a conductive connection between one. outer-electrode and said flange, means for connecting the other outer electrode to an external source of current, and a by-pass con denser connectin said other electrode to said flange.

3; A. bolometeraccording to claim 2' and including: a protective cap of. insulating material sealed over each resistor on thefront face of said disc and extending into the space between said conductor-and said casing.

4. A bolometer for coaxial transmission lines comprising, a tubular casing having a radial flange at one end and being adapted for connection to a coaxial line atthe other end, a conductor mounted at the center of. said casing, a. disc of insulated material secured to the outer face of said flange, a central electrode and two outerelectrodes formed on the inner face of said disc and providing a narrow gap between each outer electrode and the central electrode; a tempera.- ture-sensitive resistor connectedacross each gap, means providing a conductive connection be-- tween one outer electrode and saidflange', means;

for connecting the other outer electrode 623,133 external source of current, a. thin sheet oi dielec tric material. interposed. between the-central electrade and the end of said conductor,. and athini,

sheet of dielectric material interposed between said flange and the other outer electrode.

5. A bolometer resistor assembly comprising a disc of insulating material, a thin metallic coating applied to one face of said disc over a central area and two outer areas spaced from said central area by small gaps, a temperature-sensitive resistor bridging each gap, a pair of condensers each formed of a thin sheet of dielectric material having conductive coatings applied to opposite faces thereof, means securing one of said condensers in contact with the coating on said central area, and means securing said other condenser in contact with the coating on one of said outer areas.

6. A bolometer mount according to claim 5 and including a protective cap of insulating material sealed over each resistor.

WILLIAM E. WALLER. STANLEY A. JOHNSON.

References Cited in the file of this patent UNITED STATES PATENTS 10 Number 

